Wood frogs in eastern Missouri have staged a dramatic comeback.
The frogs, which had been extinct in eastern Missouri, and spotted salamanders that had been greatly reduced have come back with a flourish through a long-term conservation effort by Washington University biologists.
Because amphibians live in water and on land and are a vital link in the food chain, they are considered to be bellwether environmental species. Since the late 1980s, conservation biologists have reported alarming declines in amphibian populations in various parts of the world -- from Latin America to the North American Midwest and West Coast and parts of the deep South to a number of European countries. In some cases, apparent extinctions have occurred. However, very few of the reports or studies have documented data of more than a few years.
A team headed by Owen Sexton, Ph.D., professor emeritus of biology in Arts and Sciences, repopulated ponds at Tyson Research Center with egg masses of both wood frogs and spotted salamanders and have carefully charted their populations since 1974 for the salamanders and 1987 for the wood frogs.
The result: "Both populations are healthy and show no signs of decline," said Sexton, who is director of the center some 25 miles west of St. Louis. "I think we've shown that not all amphibian populations are at risk and that if habitats can be preserved -- if not created -- then conditions can be favorable for amphibians to thrive. I think this also shows that long-term studies are needed to get a better grasp of what may be happening to amphibian populations in other parts of the world."
Sexton and his colleagues wrote of their research in a chapter in "Status and Conservation of Midwestern Amphibians," a book edited by Michael J. Lanndo, newly published by University of Iowa Press.
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The wood frog, Rana sylvatica, is common throughout eastern and northern North America, and is characterized by a dark mask-like marking on its head. Researchers speculate that habitat loss is responsible for the wood frog disappearance and spotted salamander decline in eastern Missouri. The wood frog has disappeared throughout most of Missouri, and the creature hasn't been reported in St. Louis County for nearly 90 years.
Sexton's results are at odds with the prevailing 1990s notion that amphibian populations have declined drastically due to myriad problems such as acid rain, ozone depletion, habitat depletion and destruction, agricultural and industrial chemical pollution and even non-native predatory amphibian introductions.
And they are living proof that the adage from the popular movie "Field of Dreams" is true: "Build it, and they will come."
Under Sexton's direction, six small ponds were constructed from 1965 to 1979 in the 2,000-acre oak-hickory forest that dominates Tyson's landscape. Only one of the ponds (appropriately named Salamander Pond) had a successful introduction of spotted salamander and wood frog egg masses. However, since the original introduction, wood frogs remain today in Salamander Pond and have emigrated to one other, and spotted salamanders have emigrated to four other ponds.
The spotted salamanders were introduced from a nearby 900-acre conservation area called Forest 44. The wood frogs were introduced from a pond about 50 miles away. Wood frogs have taken so well to their new digs that they are actually spreading out from the Tyson ponds to ponds at Forest 44.
Their odyssey rivals anything Homer could have cooked up. To get to the Forest 44 ponds, the wood frogs have to negotiate Interstate-44, a six-lane highway with a 35-yard-wide grassy median.
Sexton is puzzled and amazed at the phenomenon, but draws conservation lessons from it.
"This is what's so fascinating," he said. "How they do it -- emigrate -- and what compels them to do it? I frankly have no idea. What is even more amazing is that numerous studies show amphibians are faithful to their natal, or birth, pond. They tend to return to the natal pond to breed.
"And you have to keep in mind that most of the emigrating animals are simply run over on the highway, yet the populations are thriving. I think it shows that if you release animals that genetically indicate a match for an area, then they'll disperse where habitat conditions are favorable. On the other hand, you just can't introduce populations will-nilly without suitable genetics and habitat. That would be irresponsible."
Sexton and his collaborators introduced the egg masses from other populations that are believed to be genetically similar to the spotted salamanders and wood frogs that once occupied the Tyson woods. The researchers had historical evidence from previous studies that both species were present in the area, although the last documentation of wood frogs comes from 1911.
Sexton counts the tally at the end of the spring breeding season. In 1974, the first year of the salamander census, 428 were counted, compared with 2,301 in 1995. Only two counts have been made for wood frogs; in 1987, 311 were captured, compared with 364 in 1995. There were big drops in salamander numbers in 1977 and 1978, but a big bounceback in 1986, with steadily increasing populations since then. The census is ongoing.
Sexton credits prior knowledge of genotypes, excellent habitat in the Tyson woods (providing an attractive place for amphibians to find prey) and location as factors in the repatriation success. Tyson Research Center is away from pollution sources and the forest canopy protects the amphibians from the sun's ultraviolet radiation.
